Label Distribution Protocol

The fundamental story on MPLS is that packets are labeled, and each label switching router (LSR) must perform label swapping to forward the packet. This means that in all cases, labels need to be distributed. You can achieve this in two ways: piggyback the labels on an existing routing protocol, or develop a new protocol to do just that. If you want to adjust the Interior Gateway Protocol (IGP)—such as Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP), and Routing Information Protocol (RIP)—to carry the labels, you must do it for all IGPs, because all of them are being used as routing protocols in the networks today. If you write a new protocol from the ground up, you could make it routing independent and able to work with any IGP. That is exactly the reason why Label Distribution Protocol (LDP) was invented: It carries the label bindings for the Forwarding Equivalence Classes (FECs) in the MPLS network. One exception is Border Gateway Protocol (BGP). Because BGP carries exterior routes, it is deemed more efficient if it carries the labels, too, next to the prefixes. Because BGP is already multiprotocol anyway, it can be made to carry label information with little effort. A second reason for choosing BGP to carry the label information is the fact that BGP is the only protocol distributing prefixes between autonomous systems; as such, it is a trusted protocol to function between different companies.

NOTE Multiprotocol BGP (MP-BGP) is defined in RFC 2283.

That is why the label bindings for all IGP prefixes in the routing table are distributed by LDP and all label bindings for BGP routes in the routing table are distributed by BGP in Cisco IOS. Chapter 2, "MPLS Architecture," briefly touched on LDP and exchanging label bindings. It explained why the label information base (LIB) and label forwarding information base (LFIB) are necessary and how they are created. Other basics like label operations have already been explained; this means it is necessary to explain the function and operation of LDP more in depth.

NOTE The full specification of LDP is quite lengthy. Please refer to RFC 3036, "LDP Specification," to learn the complete protocol specification, specifically the packet encodings and the procedures for advertising and processing the LDP messages.

Look at Figure 4-1, which is the network used throughout this chapter.

Figure 4-1 Network Used Throughout Chapter 4

Loopback 0 10.200.254.5/32

Figure 4-1 Network Used Throughout Chapter 4

Loopback 0 10.200.254.5/32

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